1. Technical Field
This invention relates generally to electrical circuits and in particular to rectifier devices/systems comprising an electrical transformer coupled to a multiple pulse rectifier. Still more particularly, the present invention relates to electrical circuits and designs thereof for reducing harmonic currents with rectifier devices/systems.
2. Description of the Related Art
Multiple pulse rectifiers, such as 12-pulse rectifiers, typically exhibit less than ideal characteristics when connected to an output of a transformer. One major concern with such rectifier devices/systems (i.e., a transformer with output leads coupled to a multiple pulse rectifier) is the tendency for the rectifier input to exhibit harmonic currents that also flow in the power source. These harmonic currents are a combination of lower order harmonics and higher order harmonics. This flow of harmonic currents needs to be controlled in order for the rectifier system to achieve optimal performance.
Several different transformer designs have been introduced to reduce the amount of lower order harmonic currents associated with current flowing in the power source and which is exhibited at the multi-pulse rectifier input. For example, improved transformers and/or transformer designs which enable/effect a reduction of the negative effects of lower order harmonics from the power source was introduced by U.S. Pat. No. 7,049,921. In this patent, several different designs of auto transformers are provided, which convert three phase input currents to six phase to enable a twelve-pulse (or higher) rectifier to be energized from a three phase power source. In addition to the benefit of this particular design enabling twelve pulse rectification, another significant benefit achieved by twelve pulse rectification is the substantial reduction in the 5th and 7th harmonic components of current flowing in the power source.
The harmonic current associated with twelve-pulse rectifiers (converters) is lower than with equivalent six-pulse rectifiers. However, the harmonic current associated with twelve-pulse rectifiers is often not sufficiently low for certain applications. Also, it is often necessary to mitigate higher order harmonics that are associated with rectifiers in order to meet the desired harmonic levels. Examples of such higher order harmonics are the 11th, 13th, 17th and 19th harmonics. These higher order harmonics can be mitigated by increasing the rectifier pulses to eighteen pulse, twenty-four pulse or higher. However, the use of eighteen-pulse, twenty-four-pulse or higher-pulse rectifiers causes a significant increase in the cost of the rectifier and in the cost of the associated phase shifting transformers connected thereto.
One suggestion for mitigating the higher order harmonics in these rectifier systems involves connecting inductors in series after/along the three phase power lines feeding the rectifier circuit. However, when the inductance is high enough to significantly mitigate the harmonic current, the DC voltage at the rectifier output is significantly reduced. Such reduction in DC voltage is not a desired result, and thus the use of inductors alone does not provide an acceptable design approach.